1. Field of the Invention
The present invention relates to a thin film magnetic head comprising a magneto-resistive effect element for reading as a signal a magnetic field strength of a magnetic recording medium or the like, and further relates to a head gimbal assembly and a hard disk drive each including such a thin film magnetic head.
2. Description of the Related Art
In recent years, following improvement in areal recording density of a hard disk drive, improvement in performance of a thin film magnetic head has been required. As the thin film magnetic head, use has been widely made of a composite thin film magnetic head having a structure wherein a reproducing head comprising a read-only magneto-resistive effect element (hereinafter may also be referred to simply as an “MR element”), and a recording head comprising a write-only induction-type electromagnetic transducer element are stacked relative to a substrate.
As the MR element, there can be cited an AMR element using an anisotropic magneto-resistive effect, a GMR element using a giant magneto-resistive effect, a TMR element using a tunnel-type magneto-resistive effect, or the like.
As the GMR element, a spin-valve GMR element has been often used. The spin-valve GMR element comprises a nonmagnetic layer, a soft magnetic layer formed on one surface of the nonmagnetic layer, a ferromagnetic layer formed on the other surface of the nonmagnetic layer, and a pinning layer (generally an antiferromagnetic layer) formed on the ferromagnetic layer on its side apart from the nonmagnetic layer. The soft magnetic layer is a layer that acts to change its magnetization direction depending on a signal magnetic field from the exterior. The ferromagnetic layer is a layer of which a magnetization direction is fixed by a magnetic field from the pinning layer (antiferromagnetic layer).
Large output and small Barkhausen noise are required as characteristics of the reproducing head. Generally, in order to reduce the Barkhausen noise, a bias magnetic field is applied to the MR element in a longitudinal direction (hereinafter, this bias magnetic field will be referred to as a “longitudinal bias magnetic field”). The application of the longitudinal bias magnetic field to the MR element is carried out by, for example, disposing bias magnetic field applying layers each in the form of a permanent magnet, a stacked body of a ferromagnetic layer and an antiferromagnetic layer, or the like on both sides of the MR element.
On the other hand, an increase in recording density in the hard disk drive can be achieved by reduction in track width of the thin film magnetic head, reduction in shield gap length representing a distance between two shield layers disposed on upper and lower sides of the MR element, reduction in thickness of a magnetic recording medium, reduction in size of magnetic particles contained in the magnetic recording medium, and so forth.
Following such an increase in recording density, particularly the reduction in track width, the reproduction track width of a spin-valve magneto-resistive effect film is also narrowed so that the following problem has arisen in terms of a relationship between the soft magnetic layer and the bias magnetic field applying layers.
Specifically, the narrowing of the reproduction track width entails corresponding reduction in length of the soft magnetic layer. Therefore, the bias magnetic field from the bias magnetic field applying layers provided at both ends of the soft magnetic layer becomes too strong so that the soft magnetic layer at a track center portion is inhibited from magnetization rotation, thus causing reduction in reproduction output. On the other hand, if the bias magnetic field is weakened by reducing the thickness of each bias magnetic field applying layer, stability of the magnetic head is lost.
In order to solve such a problem, JP-A-2002-367124 has proposed a magnetic head comprising a spin-valve magneto-resistive effect element having a structure wherein a single-domain forming ferromagnetic layer is formed on a soft magnetic free layer via a nonmagnetic separation layer, and the soft magnetic free layer and the single-domain forming ferromagnetic layer are magnetostatically coupled together at track width ends to form a closed magnetic circuit so that the soft magnetic free layer has magnetization substantially fixed in a direction approximately perpendicular to a magnetic field to be sensed.
However, the proposed magnetic head is insufficient in fixation of the magnetization at both end portions of the soft magnetic free layer and thus can not solve the problem about the stability of the head.
On the other hand, JP-A-2001-297412 has proposed that, in addition to normally performed longitudinal biasing, a stacked bias layer is further formed for ferromagnetic coupling or antiferromagnetic coupling to a soft magnetic layer to stabilize magnetization of the soft magnetic layer. Specifically, in this proposal, since a longitudinal bias magnetic field is weakened at a track center portion of the soft magnetic layer to thereby cause instability of a magnetic head, a bias hard magnetic film is provided on the soft magnetic layer via a nonmagnetic layer to apply a further bias magnetic field to the soft magnetic layer.
Through the ferromagnetic coupling or antiferromagnetic coupling to the soft magnetic layer, the bias hard magnetic film serves to assist the magnetization of the soft magnetic layer caused by the longitudinal bias magnetic field, further in the same direction. Accordingly, the magnetization at both end portions of the soft magnetic layer is sufficiently fixed. However, there arises a problem that the magnetic field at the track center portion of the soft magnetic layer also becomes strong like that at its end portions so that sensitivity at the track center portion decreases to lower the reproduction output.
The present invention has been conceived under these circumstances and has an object to provide a thin film magnetic head that not only ensures stable reproduction performance, but also improves sensitivity of a soft magnetic layer at a track center portion thereof to thereby improve a reproduction output, and has a further object to provide a head gimbal assembly and a hard disk drive each comprising such an improved thin film magnetic head.